1. Field of the Invention
This invention relates to a method for measuring gamma rays of trace amounts of radionuclides (radioisotopes), such as iodine-131, cobalt-60, etc., particularly in primary water of a nuclear reactor, which further contains other radionuclides such as nitrogen-13, fluorine-18, cobalt-58, etc. each emitting a pair of annihilation gamma-rays. More particularly, the invention is concerned with an improved gamma-ray spectrometric method for measuring selectively gamma-rays of the aforementioned radionuclides by diminishing the annihilation gamma-rays emitted by radionuclides coexisting in the primary water, thus significantly elevating the detection limits of the gamma-rays.
2. Statement of Related Art
For instance, at nuclear power plants, with a view toward safe operation thereof, leakage of nuclear fuel assemblies is always kept under surveillance, for example, by measuring gamma-rays of .sup.131 I .sup.60 Co ,etc. contained in trace amounts in water of a primary coolant of an individual nuclear reactor.
However, other radionuclides such as .sup.13 N, .sup.18 F, .sup.58 Co, etc., which are unstable radionuclides emitting .beta..sup.+(e.sup.+) or positron, are also contained in the primary water, and soon .beta..sup.+ decay at a low energy level by absorption in a substance and bonding with electrons therein at the end of their ranges. At that time, one positron and one electron are annihilated, emitting annihilation gamma-rays of 0.511 MeV in diametrically opposite directions.
The coexistence of the radionuclides emitting the annihilation gamma-rays is a major disturbing factor for the measurement of the intended gamma-rays, particularly gamma-ray of .sup.131 I, which has a close energy level (0.364 MeV) to the annihilation gamma-rays.
Additionally, Compton scattering caused inevitably in a gamma-ray spectrometry also interferes with the intended measurement.
As a consequence, it is essential for gamma-ray spectrometric measurement of gamma-rays of the intended radionuclides (.sup.131 I, .sup.60 Co, etc. ) in the primary water that the annihilation gamma-rays be minimized while Compton backgrounds or continua of the resulting gamma-spectra due to the gamma-rays and annihilation gamma-rays are suppressed.
Hitherto, iodine-131 and other radionuclides emitting gamma-rays in water of a primary coolant has been measured by means of a germanium (Ge) detector or a scintillation detector of NaI(T1) (sodium iodide activated by thallium) or Bi.sub.4 Ge.sub.3 O.sub.12 (bismuth germanate known as BGO), or a gamma-ray sepctrometric measurement system wherein a scintillation detector is disposed around a germanium detector. The method using the Ge detector was poor in detection limit of .sup.131 I owing to the effect of Compton backgrounds produced from .sup.131 I gamma-ray, .sup.60 Co gamma-ray the annihilation gamma-rays, etc., so that trace amounts of .sup.131 I and other radionuclides emitting gamma-rays in the primary water couldn't be measured. Only in the event that .sup.131 I, .sup.60 Co, etc. were leaked from a fuel assembly into the primary water, increased concentrations of them enabled the measurement.
Again, the method using the NaI(T1) detector was too inferior to the Ge detector method in resolution power.
The method using both Ge detector and scintillation detector has been improved more or less over the preceding methods, but it has still not been possible to measure extremely slight concentrations of .sup.131 I, .sup.60 Co and others.
Thus, any of the known gamma-ray spectrometric methods has not been satisfactory and feasible because the annihilation gamma-rays from coexsiting radionuclides in the primary water have interfered with the measurement of the intended radionuclides, e.g., .sup.131 I, etc.
Another method for measuring .sup.131 I and other radionuclides by chemical analysis has been known, but has yielded disadvantageously awkward radioactive wastes, which should be handled or disposed of with great care. Hence, this is not suitable for frequent or continuous measurement.
In view of the drawbacks or problems as encountered in the prior art measurement methods of gamma-rays in primary water of a nuclear reactor as stated above or gamma-rays in another radioactive substances, the present invention is designed to provide a gamma-ray spectrometric measurement method which enables to significantly enhance detection limits of gamma-ray-emetting radionuclides, particularly in the primary water.
That is to say, it is a primary object of the invention to provide an improved method for measuring selectively gamma-rays of radionuclides (iodine-131, cobalt-60, etc.), particularly in the primary water contained in micro-quantities by excluding disturbing factors to the measurement, namely, the aforesaid annihilation gamma-rays emitted by other radionuclides coexisting in the primary water, and Compton effects due to the gamma-rays and annihilation gamma rays as far as possible.
Another object of this invention is to provide a high-sensitive measurement method capable of detecting such extremely slight amounts of the radionuclides emitting gamma rays in the primary water that it has been not possible to detect hitherto.
Further object is to provide a reliable measurement method which enables continuous surveillance of leakage of a nuclear fuel assembly, thereby assisting in early prevention of the risk.